1. Field of the Invention
The present invention relates to sheet or film-forming rolls, sheet or film-forming apparatus and crowning control methods, and more particularly, relates to a touch roll and a crowning control method suitable for forming thin sheets or films.
2. Description of the Related Art
There has been known a touch roll type method for continuously forming a sheet or film by supplying melting resin between a pair of opposed rolls, a main roll and a sub roll (touch roll), from a T-die. In general, inside the touch roll heating medium is circulated to regulate a surface temperature thereof.
In this kind of sheet or film-formation, there is a touch roll whose outer surface is flat in its axial direction. In forming a relatively thick sheet or film, even though pressure applied on the touch roll is increased, the touch roll keeps contacting the melting resin in its full width. As a result, a flat sheet or film is formed.
In forming a thin sheet or film, however, as the pressure is increased, the touch roll bends in an arch-like shape making a gap against the main roll at a center of the touch roll, thereby reducing the pressure at a center of the touch roll and bringing a lack of contact. As a result, a non-flat sheet or film is formed.
Japanese Patent Publication Laid-open No. 2002-36332 discloses a touch roll having a double structure of an outer cylinder and an inner cylinder. The outer cylinder has a metallic thin-walled structure with a predetermined amount of crowning shaped thereon not to make the gap.
In actual sheet or film-forming, however, thickness of a film to be formed or kind of resin to be used is various. Accordingly an appropriate pressure to be applied on the touch roll changes as well as an appropriate amount of crowning changes. The touch roll thus lacks flexibility.
Japanese Patent Publication No. 3422798 (referred to as “D2” below) discloses a touch roll (the first roll) having a metallic thin-walled cylinder (see FIG. 1). An amount of crowning to be given on the first roll is adjusted by controlling pressure on heating medium (cooling water) supplied inside the metallic thin-walled cylinder. The first roll has a problem that the amount of crowning becomes instable. Given on the heating medium circularly, the pressure is easy to fluctuate. In fact, the heating medium is used for two purposes: (1) controlling its temperature for cooling; and (2) giving pressure thereon for crowing. Another problem is a limit on pressure capacity of sealing members, which is 450-600 kPa at most in which increasing the pressure causes seal leakage.
“D2” discloses another touch roll (the second roll) with an eccentric double structure of an outer metallic thin-walled cylinder and an inner cylinder (see FIG. 17). The inner cylinder has an elastic surface contacting an inner surface of the outer metallic thin-walled cylinder by fixing the inner cylinder eccentrically to the outer metallic thin-walled cylinder. Since an amount of crowning to be given on the second roll is adjusted by controlling pressure on heating medium (cooling water) supplied inside the outer metallic thin-walled cylinder, the second roll has the same problems as the first roll, the instability of crowning and the limitation of pressure capacity of sealing.
“D2” discloses another touch roll (the third roll) having a concentric double structure (see FIGS. 6-8). Both cylinders are filled with medium: an outer cylinder with crowning medium and an inner cylinder with cooling medium. The third roll has another problem in controlling an outer surface temperature because the cooling medium is away from the outer surface.
Japanese Patent Publication Laid-open No. 2000-506795 discloses another touch roll with a concentric double structure of an outer cylinder filled with cooling medium and an inner cylinder filled with crowning medium. The touch roll has a limit on an amount of crowning because the crowning medium is away from an inner surface of the outer cylinder.